Please take a look at this...

I think my ph was off for a while. I tested it and it was around 4.5 so that could really be the reason. But the ph has not always been off and the plants show several different symptoms such as soft, dark green leaves, dry leaves, completely yellow leaves that are also were soft and just hanging down and leaves with brown spots and brown tips.

I would also like to know (just by looking at the leaves) whether I might be using too much fert or too little. I dont have an EC meter.

Your help is appreciated.

Specs:

400W HPS
Hydroponics (air pumps)
Holland's Secret nutrient set
Strains: Northern Lights, Orange Kush, Trainwreck, and two dank bag seeds

A ph. of 4.5 locks out all kinds of nutrients so does a ph of 5. You're going to need to correct the ph. before you can solve any nute probs. The ph. can change sereral times a day, so check it as frequently as possible.

I guess it has to be the ph....

What about magnesium deficiency though, could that have something to do with that?

Sure, but any magnesium deficiency is caused by, and won't be corrected until you change your ph.